1. Field of the Invention
The present invention relates to a voltage controlled oscillator.
2. Description of the Related Art
A conventional frequency synthesizer using a voltage controlled oscillator comprises a reference oscillator 1, a frequency divider 5 for dividing a frequency by M (f0/M) such that an oscillating frequency f0 of a voltage controlled oscillator 4 is equal to an oscillating frequency fr of the reference oscillator 1, a phase comparator 2 for comparing the output phase of the reference oscillator 1 with the output phase of the frequency divider 5 to detect a phase difference, and a low-pass filter 3 for removing unnecessary harmonic wave components included in the output of the phase comparator 2, as shown in FIG. 5.
In such a structure, a feedback function that the phase difference between fr and f0/M becomes always zero is performed, and the output phase of the voltage controlled oscillator 4 is synchronized with the output phase of the reference oscillator 1, thereby obtaining high stability oscillation in accordance with the phase precision of the reference oscillator 1.
In the conventional example, the harmonic wave components (2f0, 3f0,) of a high order and a fundamental wave component f0 are simultaneously output from the voltage controlled oscillator 4. Therefore, a band-pass filter 6 selects the fundamental wave, and a multiplier 7 multiplies the output of the band-pass filter 6 by N, thereby obtaining a desired oscillating frequency N×fo (for example, see Japanese unexamined Patent Application Publication No. 10-126262 (FIG. 5).
In the conventional structure, the voltage controlled oscillator outputs the fundamental wave and the harmonic wave from only one output terminal thereof and oscillates with a frequency lower than a predetermined frequency. Also, the multiplier generates an oscillating signal having a desired frequency by using the fundamental wave output from the voltage controlled oscillator. Therefore, the band-pass filter and the multiplier are required. As a result, the structure of the frequency synthesizer becomes complicated, and thus the miniaturization thereof cannot be accomplished.